Recently, electronic devices have been rapidly becoming smaller, lighter, and cordless. This tendency is remarkable in information electronic appliances, such as personal computers, cellular phones, and PDAs, and audio-visual electronic appliances, such as video camcorders and mini-disc players.
Batteries, especially secondary batteries with high energy density are desired as power sources for such electronics devices. Among them, non-aqueous electrolyte secondary batteries provide high energy densities incomparable to those of lead-acid batteries, nickel-cadmium storage batteries and nickel-metal hydride storage batteries. Thus, non-aqueous electrolyte secondary batteries are becoming dominant as a power source for such electronic devices.
Non-aqueous electrolyte secondary batteries (e.g., lithium ion secondary batteries and lithium ion polymer secondary batteries) include a positive electrode active material that provides an average discharge potential of 3.5 V to 4.0 V relative to lithium. Exemplary positive electrode active materials include lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium manganate (LiMn2O4), mixtures thereof, and solid solution materials containing a plurality of transition metals (e.g., LiCoxNiyMnzO2, Li(CoaNibMnc)2O4).
Such a positive electrode active material is mixed with a conductive agent and a binder, to form a positive electrode mixture. The positive electrode mixture is applied to a current collector or a case serving as a current collector. Alternatively, the mixture is compression-molded into a predetermined shape. The current collector or case serving as the current collector is composed of aluminum, titanium or stainless steel.
As a negative electrode active material, carbon material capable of absorbing and desorbing lithium is preferably used. Exemplary carbon materials include artificial graphite, natural graphite, heat-treated mesophase material made from coal or petroleum pitch, and amorphous carbon.
Such a negative electrode active material is mixed with a binder and the like, to form a negative electrode mixture. The negative electrode mixture is applied to a current collector or a case or a cap serving as a current collector. Alternatively, the mixture is compression-molded into a predetermined shape and then fixed into the case or the cap composed of iron or nickel preferably. The current collector is preferably composed of copper foil.
When a graphite material is used as the negative electrode active material, the average potential at which lithium ions are released is approximately 0.2 V lower than that when an amorphous carbon is used. Thus, a graphite material is suitable as the negative electrode active material in case high voltage and voltage plateau flatness are desired.
A non-aqueous electrolyte is selected so as to resist the oxidizing atmosphere of the positive electrode that discharges at such high potentials as described above and the reducing atmosphere of the negative electrode that charges and discharges at potentials close to that of lithium. A currently used non-aqueous electrolyte is composed of a solvent mixture of ethylene carbonate (EC) with a high dielectric constant and a chain carbonate (acyclic carbonate) with a low viscosity, and lithium hexafluorophosphate (LiPF6) dissolved therein. As the chain carbonate, for example, one or more of diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are used. Cyclic carbonates and cyclic esters with high dielectric constants, such as propylene carbonate and γ-butyrolactone, are also used. As the low-viscosity solvent, for example, fluorobenzene is used.
Lithium ion polymer secondary batteries employ a gel polymer as an electrolyte. The gel electrolyte comprises, for example, the above-described non-aqueous electrolyte as a plasticizer retained in a polymer component.
In order to improve the high temperature storage characteristics and cycle life characteristics of non-aqueous electrolyte secondary batteries, it has been proposed to add vinylene carbonate, propanesultone, phenyl ethylene carbonate, vinyl ethylene carbonate, or the like to the non-aqueous electrolyte. Also, in order to enhance the safety during overcharge, it has been proposed to add the following materials in some cases, for example, cyclohexyl benzene, biphenyl benzene, or diphenyl ether to the non-aqueous electrolyte.
The capacity of a non-aqueous electrolyte secondary battery is limited by the maximum capacity density of a positive electrode (approximately 282 mAh/g for LiCoO2) and the maximum capacity density of a negative electrode (approximately 372 mAh/g for graphite). Therefore, standardized batteries whose volumes are limited cannot provide so much improvement in energy density as long as they utilize conventional materials or materials similar to conventional ones.
Under such conditions, a secondary battery with a new structure is being studied which utilizes sodium in a negative electrode, a molten salt in a positive electrode, and a sodium-ion-conductive solid electrolyte in a separating film (Matsunaga, entitled “Behavior of Na/Se(IV) secondary battery utilizing AlCl3—NaCl molten salt”, DENKI KAGAKU, 1983, Vol. 51, No. 10, p. 847-848).